XF 1.2 Retroactively apply signature permissions?

I want to take advantage of the new user group signature permissions when I switch over to XenForo, but they obviously aren't going to apply to users until they edit their signatures. Is there any way to retroactively enforce these new rules?

Assuming my presumptions are correct (that it's not possible), what would be the most efficient way to purge all user signatures from the database? Should I just run a manual query?